1. Field of the Invention
The present invention relates to a sheet feeding method and a sheet feeding device for image forming apparatuses, such as copying machines, printers, facsimile apparatuses, and printing apparatuses, and also relates to an image forming apparatus using the sheet feeding device.
2. Discussion of the Background
In image forming apparatuses, such as copying machines, printers, facsimile apparatuses, and printing apparatuses, sheets to be printed on are stacked in a sheet feeding part of the apparatuses and are separated one by one by a sheet feeding device of the apparatuses so as to be fed to an image forming part of the apparatuses. Known sheet feeding devices include feed and reverse rollers (FRR) type device, a friction roller (FR) type device, and a friction pad (FP) type device.
Recently, with the increase in the use of color images, a coated sheet having a superior smoothness has been widely used for sheets to be printed on in image forming apparatuses for obtaining a better image quality. The coated sheets tend to closely contact each other, either because of the smoothness of their surfaces or under the influence of humidity, in a sheet feeding part of image forming apparatuses, and thereby incomplete separation of the sheets occurs, resulting in double feeding of the sheets.
The following proposals are known with respect to improvement of sheet separation performance of sheet feeding devices of image forming apparatuses:
a) Japanese Patent Laid-Open Publication No. 5-201571 relates to a sheet feeding device which includes a feed roller rotating at a constant position and a separation member contacting the feed roller and in which sheets are fed into a nip between the feed roller and the separation member. For increasing the sheet separation performance, a vibrating member is arranged so as to contact the separation member at the backside thereof, and the separation member is vibrated by the vibrating member back and forth in a sheet feeding direction.
b) Japanese Patent Laid-Open Publication No. 5-213468 discloses that a mechanism for generating a force to stop conveyance of a sheet (i.e., the mechanism serving as a separation member) is elastically brought into contact with a rotating member (i.e., serving as a feed roller) rotating at a constant position. The separation member is vibrated by piezoelectric ceramics, so that the force to stop conveyance of a sheet by the separation member is freely suppressed and thereby, the sheet separation performance, corresponding to a change in the quality of sheets to be printed on, is obtained.
c) Japanese Patent Laid-Open Publication No. 5-330683 relates to a sheet feeding device in which a friction pad contacts a feed roller rotating at a constant position. For improving the separation performance, the friction pad is vibrated by a piezo-electric element so that the pressure of the friction pad is suppressed and the vibration is transmitted to the sheets to be printed on.
d) Japanese Patent Laid-Open Publication No. 6-100179 proposes to provide vibration, for increasing the sheet separation performance, to stacked sheets in a sheet feeding tray so that the stacked sheets are loosened.
In the above-described proposals a), b) and c), the separation member, which does not directly contact the sheet to be separated from the other sheets so as to be fed, is vibrated, and therefore the vibration is indirectly applied to the sheet to be separated in a thickness direction of the stacked sheets. Thereby, the sheet to be separated is not sufficiently vibrated, resulting in incomplete separation of the sheet. Also, in the above-described proposal d), the vibration is applied to the feeding tray, so that the sheet to be separated from the others to be fed is not directly vibrated, thereby resulting in incomplete separation of the sheet. Further, the vibration of the separation member by a high frequency wave does not provide the effect of the vibration over the entire part of a sheet. Therefore, the effect of loosening the stacked sheets is not sufficient to prevent non-feeding of the sheets.